python,python3: add support for SOURCE_DATE_EPOCH var

See:
https://github.com/openwrt/packages/issues/5278

This should make Python & Python3 packages reproducible
when building.
In my local tests, I got the same sha256 for a sample
.pyc file, so likely this is the solution that should address
this.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/python3/patches/012-add-support-source-date-epoch-pyc.patch

@@ -0,0 +1,13 @@
+--- a/Lib/py_compile.py
++++ b/Lib/py_compile.py
+@@ -137,8 +137,9 @@ def compile(file, cfile=None, dfile=None
+     except FileExistsError:
+         pass
+     source_stats = loader.path_stats(file)
++    mtime = int(os.getenv('SOURCE_DATE_EPOCH', source_stats['mtime']))
+     bytecode = importlib._bootstrap_external._code_to_bytecode(
+-            code, source_stats['mtime'], source_stats['size'])
++            code, mtime, source_stats['size'])
+     mode = importlib._bootstrap_external._calc_mode(file)
+     importlib._bootstrap_external._write_atomic(cfile, bytecode, mode)
+     return cfile